Along with the continuing demand for new materials with improved high temperature performance, there has been strong interest, most notably for aerospace systems, in developing high temperature materials of low density and high strength to density ratios for reasons of improved efficiency and economy. It is to be noted that, as discussed in "Superalloys--A Technical Guide" by Elihu F. Bradley, ed., ASM International, Metals Park, OH (1988), common high temperature alloys have densities of the order of 8 g/cc. Those densities are more than twice the densities of the alloys presented by this invention.
The low density binary aluminum-titanium intermetallic alloy Al.sub.3 Ti is known to have high strength, high hardness (.about.450 HDP), as well as good heat and oxidation resistance, but is extremely brittle at room temperature. M. Yamaguchi, Y. Umakoshi and T. Yamane in "Philosophical Magazine" A, 55 (1987) 301, discuss this phenomenon. Some attempts to enhance Al.sub.3 Ti type alloys for increased utilization have been in the area of investigations of processing technology. However, the prospects for improving the ductility by processing methods are poor, primarily because of the tetragonal (DO.sub.22) crystal structure, which has less than the requisite number of slip systems required for polycrystalline deformation and ductility. Also, the binary alloys are difficult to prepare. Other aluminum-based alloys of the type Al.sub.3 X, where X represents elements from Groups IVA and VA of the periodic table, e.g., V, Zr, Nb, Hf and Ta, are known to have similar characteristics. The A subgroup designation used herein is that recommended by the International Union of Pure and Applied Chemistry, wherein Group IVA is headed by Ti, Group VA is headed by V and Group VIA is headed by Cr.
It is well known that alloys with the cubic crystal structure (Ll.sub.2) can be more ductile at low temperatures because they possess the requisite number of slip systems. These alloys also often exhibit a positive temperature dependence of compressive strength.
It has been known for some time that tetragonal Al.sub.3 Ti can be transformed to the cubic Ll.sub.2 structure by ternary addition of Fe, Cu, or Ni. That phenomenon is discussed in the publications: A. Raman and K. Schubert, Z. Metallk, 56 (1965) 99; A. Seibold, Z. Metallk, 72 (1981) 712; and K. S. Kumar and J. R. Pickens, Scripta Met. 22 (1988) 1015. As a specific example, Kumar and Pickens, "Ternary Low-Density Cubic Ll.sub.2 Aluminides," Proceedings of the Symposium Dispersion Strengthened Aluminum Alloys, 1988 TMS Annual Meeting, Phoenix, Ariz., Jan. 25-28, 1988 summarize some of these earlier observations, and describe cubic versions of the alloys Al.sub.5 CuTi.sub.2 and Al.sub.22 Fe.sub.3 Ti.sub.8. Reported hardnesses were .about.330 HDP, with the alloys showing little resistance to cracking in the vicinity of test hardness indentations. In general, alloys of this type have been difficult to produce, suffering from porosity, inhomogeneity, and second phases, all of which can have deleterious effects on mechanical properties. There are also indications that additions of Cu or Fe decrease the resistance to oxidation at high temperatures.